Miniature sensor devices can be micromachined in single crystal silicon (SCS) wafers using a variety of techniques common in the integrated circuit manufacturing industry. SCS is advantageously end extensively used in integrated circuits because, in addition to having the necessary electrical properties, it possesses high yield strength, elastic modulus, and fatigue strength. Still further, SCS allows for signal conditioning, amplifying, and control electronics to be fabricated on a single chip with a transducer. This last feature may lead to simplification and cost advantages in fabrication, assembly, and installation, and may also minimize signal loss, stray signals, and noise between the device and the circuitry due to connecting lead length. The silicon would be processed in the conventional manner for integrated circuit production, with the extra structural shape for the transducer being micromachined in after the electronic devices are incorporated.
Examples of transducer structures which may be micromachined from single crystal silicon are pressure sensor diaphragms (whose deflection, due to pressure differences, may be sensed by piezoresistive strain gauges diffused or implanted into the diaphragm surface or by capacitance changes between the diaphragm and the supporting structure), accelerometer cantilevers with diffused or implanted piezoresistors to sense deflection, and diaphragms or beams for thermal sensors, e.g., infrared thermopile detectors or thermal vacuum sensors.
Even though new micromachining techniques--and novel applications of old techniques--are now being continuously developed for producing such micro-mechanical structures, the most powerful and most versatile micromachining tool continues to be etching. Etching techniques for silicon include chemical, plasma, reactive-ion, and sputter etching. Chemical etchants for silicon are numerous. They can be isotropic or anisotropic, dope independent or not, and have varying degrees of selectivity to silicon, which determines the appropriate masking material. In order to micromachine silicon by chemical etching, the silicon is provided with a mask, such as photolithographically patterned silicon dioxide, and then the silicon is exposed to an etching solution, whereby the unmasked portion is etched. SCS wafers advantageously possess the ability to be anisotropically etched in depth with minimal lateral undercutting of the patterned mask. The thickness of the silicon product being formed by etching is controlled by regulating the time the silicon is left in the etchant, the etch rate of the etchant being known. Various approaches are known to increase the etch rate of the etching solution. For example, it has been shown by R. M. Finne and D. L. Klein in "A Water-Amine-Complexing Agent System for Etching Silicon", in J. Electrochem. Soc.: Solid State Science, September 1967, Vol. 14, No. 9, pgs. 965-970 that the etch rate of a commonly used etching solution comprising ethylenediamine, pyrocatechol, and water (EDP) varies with the water and pyrocathechol content. This reference teaches that the etch rate is also a function of the temperature of the etching solution, an increased temperature producing an accelerated etch rate. Still further, A. Reisman et al. teach in "The Controlled Etching of Silicon in Catalyzed Ethylenediamine-Pyrocatechol-Water Solutions", J. Electrochem. Soc.: Solid-State Science and Technology, August 1979, Vol. 126, No. 8, pgs. 1406-1415 that the etch rate in such etching solutions can be enhanced by the addition of trace quantities of 1,4- and 1,2-diazine. U.S. Pat. No. 4,155,866 to Berkenblit et al. is directed to that invention.